Not only is the Chesapeake Bay so enormous it can be seen from space, it essentially came from outer space.

An asteroid or huge chunk of ice slammed into Earth about 35 million years ago, splashing into the North Atlantic Sea, sending tsunamis as far as the Blue Ridge Mountains and leaving a 56-mile-wide hole at the mouth of what is now the bay.

But a newly published research paper written by U.S. Geological Survey scientists shows that wasn’t the end of it. While drilling holes in southern Virginia to study the impact crater, the scientists discovered “the oldest large body of ancient seawater in the world,” a survivor of that long gone sea, resting about a half-mile underground near the bay, according to the USGS.

“What we essentially discovered was trapped water that’s twice the salinity of [modern] seawater,” said Ward Sanford, a USGS hydrologist. “In our attempt to find out the origin, we found it was early cetaceous seawater. It’s really water that’s from the North Atlantic.”

The findings showing that the water is probably between 100 million and 150 million years old were published Thursday in the journal Nature.

The Chesapeake Bay Impact Crater was discovered in 1999 by a tandem of USGS and Virginia Department of Environmental Quality scientists.

They theorized that a huge rock or chunk of ice slammed into an ancient ocean, sending enormous pieces of debris skyward and forcing monster tsunamis hundreds of miles inland.

Over centuries, the crater became hidden under 400 to 1,200 feet of sand, silt and clay, hampering its discovery for decades.

“It’s the largest crater discovered so far in the United States, and it’s one of only a few oceanic impact craters that have been documented worldwide,” USGS hydrologist David Powars said at the time.

The bay crater is shallower and smaller than another off the coast of Mexico, which most scientists believe caused the extinction of dinosaurs, Powars said.

Five years after the Chesapeake crater’s discovery, Sanford’s USGS team started drilling at Cape Charles, Va., under a $1.5 million grant from the International Continental Drilling Program to study how the earth’s crust absorbed the blow. “We weren’t looking for ancient seawater,” he said.

As the team drilled a half-mile from the surface, it encountered standing water. They first thought it was salty water that occasionally shows up at coastal drill sites. Saltwater is found underground all over the world all the time, often because of huge salt deposits in the ground.

In this case, “we didn’t hit any salt while drilling” at the Cape Charles site, a mile from the bay, Sanford said.

So researchers considered the possibility of boiling, when a meteor impact is so forceful that it heats water and increases its salinity. But after further tests, the boiling theory also didn’t make sense.

Results from more testing showed the water was twice as salty as today’s ocean water. When they analyzed its chemistry, they found high levels of chloride and bromide, the fingerprint of seawater from another time, Sanford said.

More tests and digging through research established that the chemistry was consistent with the “vast halite deposits created during the Jurassic and Cretaceous periods in the Gulf of Mexico and South Atlantic Basins,” the research paper said.

In other words, the groundwater at Cape Charles, about 220 miles south of the District of Columbia, had the same salinity as the long-gone North Atlantic sea. When the meteor or whatever it was struck North America and disfigured the landscape, “the ancient seawater was preserved like a prehistoric fly in amber,” the USGS said in a statement.

“This was a surprising site discovery,” Sanford said. “This was not something we were looking for or even expected.”

It wasn’t the first time teams drilling for oil or geologic studies have come across deep groundwater with strangely high salinity, said Jerad Bales, acting USGS associate director for water.

Bales said numerous theories had previously been offered to help explain those findings. “But, up to this point, no one thought that this was North Atlantic Ocean water that had essentially been in place for about 100 million years,” Bales said.

The USGS said the discovery would help scientists to better understand the hydrology of the area, at the very least.

Fourteen years ago, when the crater itself was found, Bales made a prescient statement. He said the discovery would help explain a few strange features in the region, including earthquakes around the crater’s perimeter, a higher rate of sea-level rise around Norfolk, Va. and “salty groundwater.”